Dual function prosthetic bone implant and method for preparing the same

ABSTRACT

The present invention discloses a prosthetic bone implant made of a hardened calcium phosphate cement having an apatitic phase as a major phase, which includes a dense cortical portion bearing the majority of load and a porous cancellous portion allowing a rapid blood/body fluid penetration and tissue ingrowth.

PRIORITY CLAIM

This application claims priority to Non-Provisional patent application Ser. No. 10/852,167 entitled “DUAL FUNCTION PROSTHETIC BONE IMPLANT AND METHOD FOR PREPARING THE SAME” filed on May 25, 2004 now U.S. Pat. No. 6,994,726.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a prosthetic bone implant made of a hardened calcium phosphate cement having an apatitic phase as a major phase, and in particular to a prosthetic bone implant comprising a dense cortical portion bearing the majority of load and a porous cancellous portion allowing a rapid blood/body fluid penetration and tissue ingrowth.

2. Description of the Related Art

It is advantageous if a prosthetic bone implant is bioresorbable and is supportive at the same time. Accordingly, an article made of calcium phosphate will be preferable than that made of a metal, if the former has strength which is comparable to a human cortical bone. One way of making such a bone implant is by sintering a calcium phosphate powder, particularly a hydroxyapatite (HA) powder, into a block material at a temperature generally greater than 1000° C. Despite the fact that the high temperature-sintered HA block material has an enhanced strength, the bioresorbability of the material is largely sacrificed, if not totally destroyed, due to the elimination of the micro- and nano-sized porosity during the sintering process.

The conventional spinal fusing device is composed of a metallic cage and a bioresorbable material disposed in the metal cage, for example the one disclosed in U.S. Pat. No. 5,645,598. An inevitable disadvantage of this fusion device is the sinking of the metallic cage sitting between two vertebrae to replace or repair a defect spinal disk, because the hardness and the relatively small size of the cage wear out or break the bone tissue, and in particular the endplate of the vertebra.

SUMMARY OF THE INVENTION

A primary objective of the invention is to provide a prosthetic bone implant free of the drawbacks of the prior art.

The prosthetic bone implant constructed according to the present invention is made of a hardened calcium phosphate cement having an apatitic phase as a major phase, which comprises a dense cortical portion bearing the majority of load and a porous cancellous portion allowing a rapid blood/body fluid penetration and tissue ingrowth.

The prosthetic bone implant of the present invention is made by a novel technique, which involves immersing an article molded from two different pastes of calcium phosphate cement (CPC), one of them having an additional pore-forming powder, in a liquid for a period of time, so that the compressive strength of the molded CPC article is significantly improved after removing from the liquid while the pore-forming powder is dissolved in the liquid, creating pores in a desired zone or zones of the molded article.

Features and advantages of the present invention are as follows:

-   -   1. Easy process for different shape and size of the prosthetic         bone implant of the present invention, so that the outer         circumferential dense portion thereof can sit over the         circumferential cortical portion of a bone and the porous         portion thereof can contact the cancellous portion of the bone         adjacent to a bone receiving treatment.     -   2. The dense cortical portion of the prosthetic bone implant         made according to the present invention exhibits a high strength         comparable to that of human cortical bone (about 110–170 MPa).         The strength is adjustable by adjusting process parameters.     -   3. The dense cortical portion of the prosthetic bone implant         made according to the present invention contains significant         amount of micro- and nano-sized porosity, that improves         bioresorbability thereof. Conventional high temperature-sintered         HA block, on the other hand, does not possess sufficient         micro/nano-sized porosity and is not bioresorbable.     -   4. The porous cancellous portion of the prosthetic bone implant         made according to the present invention possesses a porosity         greater than 40% in volume, prepferably 40–90%, allowing rapid         blood/body fluid penetration and tissue ingrowth, thereby         anchoring the prosthetic bone implant.     -   5. A wide range of medical application includes bone dowel,         spacer, cavity filler, artificial disc and fixation devices for         spine and other locations, to name a few.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 d show schematic cross sectional views of four different designs of prosthetic bone implants constructed according to the present invention.

FIGS. 2 a to 2 f are schematic cross sectional views showing steps of a method for preparing a prosthetic bone implant according to one embodiment of the present invention.

FIGS. 3 a and 3 b are schematic vertical and horizontal cross sectional views of a prosthetic bone implant prepared according to another embodiment of the present invention, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention includes (but not limited to) the following:

-   1. A prosthetic bone implant comprising a cortical portion having     two opposite sides, and a cancellous portion integrally disposed in     said cortical portion and being exposed through said two opposite     sides, wherein said cortical portion comprises a hardened calcium     phosphate cement has a porosity of less than 40% in volume, and said     cancellous portion comprises a porous hardened calcium phosphate     cement having a porosity greater than 20% in volume, and greater     than that of said cortical portion. -   2. The implant according to Item 1, wherein the cortical portion is     in the form of a hollow disk, and the cancellous portion is in the     form of a column surrounded by the hollow disk. -   3. The implant according to Item 2 further comprising a transitional     portion between said column and said hollow disk surrounding said     central cylinder, which has properties range from those of said     cancellous portion to said cortical portion. -   4. The implant according to Item 1, wherein the cortical portion is     in the form of a disk having one or more longitudinal through holes,     and the cancellous portion is in the form of one or more columns     surrounded by said one or more longitudinal through holes. -   5. The implant according to Item 1, wherein said hardened calcium     phosphate cement of said cortical portion comprises an apatitic     phase as a major phase giving rise to broadened characteristic X-ray     diffraction peaks in comparison with a high-temperature sintered     apatitic phase. -   6. The implant according to Item 5, wherein said broadened     characteristic the X-ray diffraction peaks are at 2-Theta values of     25–27° and 30–35°. -   7. The implant according to Item 1, wherein said hardened calcium     phosphate cement of said cortical portion is prepared without a high     temperature sintering. -   8. The implant according to Item 1, wherein said hardened calcium     phosphate cement of said cortical portion comprises an apatitic     phase as a major phase having a Ca/P molar ratio of 1.5–2.0. -   9. The implant according to Item 1, wherein said hardened calcium     phosphate cement of said cancellous portion comprises an apatitic     phase as a major phase giving rise to broadened characteristic X-ray     diffraction peaks in comparison with a high-temperature sintered     apatitic phase. -   10. The implant according to Item 9, wherein said broadened     characteristic the X-ray diffraction peaks are at 2-Theta values of     25–27° and 30–35°. -   11. The implant according to Item 1, wherein said hardened calcium     phosphate cement of said cancellous portion is prepared without a     high temperature sintering. -   12. The implant according to Item 1, wherein said hardened calcium     phosphate cement of said cancellous portion comprises an apatitic     phase as a major phase having a Ca/P molar ratio of 1.5–2.0. -   13. The implant according to Item 1, wherein said cortical portion     comprises 10–90% in volume of said implant. -   14. The implant according to Item 1, wherein said cortical portion     has a porosity of less than 30% in volume. -   15. The implant according to Item 1, wherein said cancellous portion     has a porosity greater than 40–90% in volume. -   16. A method for preparing a prosthetic bone implant comprising a     cortical portion having two opposite sides, and a cancellous portion     integrally disposed in said cortical portion and being exposed     through said two opposite sides, said method comprises the following     steps:     -   a) preparing a first paste comprising a first calcium phosphate         cement and a first setting liquid;     -   b) preparing a second paste comprising a second calcium         phosphate cement, a pore-forming powder and a second setting         liquid;     -   c) i) preparing a shaped article in a mold having two or more         cells separated by one more partition walls comprising         introducing said first paste and said second paste into said two         or more cells separately, and removing said one or more         partition walls from said mold, so that said second paste in the         form of a single column or two or more isolated columns is         integrally disposed in the first paste in said mold; or ii)         preparing a shaped article comprising introducing one of said         first paste and said second paste into a first mold to form an         intermediate in said first mold, placing said intermediate into         a second mold after a hardening reaction thereof undergoes at         least partially, and introducing another one of said first paste         ad said second paste into said second mold, so that said second         paste as a single column or as two or more isolated columns is         integrally disposed in the first paste in said second mold;     -   d) immersing the resulting shaped article from step c) in an         immersing liquid for a first period of time so that said         pore-forming powder is dissolved in the immersing liquid,         creating pores in said single column or said two or more         isolated columns; and     -   e) removing the immersed shaped article from said immersing         liquid. -   17. The method according to Item 16 further comprising     -   f) drying the immersed shaped article. -   18. The method according to Item 16, wherein said pore-forming     powder is selected from the group consisting of LiCl, KCl, NaCl,     MgCl₂, CaCl₂, NaIO₃, KI, Na₃PO₄, K₃PO₄, Na₂CO₃, amino acid-sodium     salt, amino acid-potassium salt, glucose, polysaccharide, fatty     acid-sodium salt, fatty acid-potassium salt, potassium bitartrate     (KHC₄H₄O₆), potassium carbonate, potassium gluconate (KC₆H₁₁O₇),     potassium-sodium tartrate (KNaC₄H₄O₆.4H₂O), potassium sulfate     (K₂SO₄), sodium sulfate, and sodium lactate. -   19. The method according to Item 16, wherein said first calcium     phosphate cement comprises at least one Ca source and at least one P     source, or at least one calcium phosphate source; and said second     calcium phosphate cement comprises at least one Ca source and at     least one P source, or at least one calcium phosphate source. -   20. The method according to Item 19, wherein said first calcium     phosphate cement comprises at least one calcium phosphate source,     and said second calcium phosphate cement comprises at least one     calcium phosphate source. -   21. The method according to Item 20, wherein said calcium phosphate     source is selected from the group consisting of alpha-tricalcium     phosphate (α-TCP), beta-tricalcium phosphate (β-TCP), tetracalcium     phosphate (TTCP), monocalcium phosphate monohydrate (MCPM),     monocalcium phosphate anhydrous (MCPA), dicalcium phosphate     dihydrate (DCPD), dicalcium phosphate anhydrous (DCPA), octacalcium     phosphate (OCP), calcium dihydrogen phosphate, calcium dihydrogen     phosphate hydrate, acid calcium pyrophosphate, anhydrous calcium     hydrogen phosphate, calcium hydrogen phosphate hydrate, calcium     pyrophosphate, calcium triphosphate, calcium phosphate tribasic,     calcium polyphosphate, calcium metaphosphate, anhydrous tricalcium     phosphate, tricalcium phosphate hydrate, and amorphous calcium     phosphate. -   22. The method according to Item 21, wherein said first calcium     phosphate cement and said second calcium phosphate cement are     identical. -   23. The method according to Item 22, wherein said first calcium     phosphate cement and said second calcium phosphate cement are     tetracalcium phosphate. -   24. The method according to Item 16, wherein the first setting     liquid and the second setting liquid independently are an acidic     solution, a basic solution, or a substantially pure water. -   25. The method according to Item 24, wherein said acidic solution is     selected from the group consisting of nitric acid (HNO₃),     hydrochloric acid (HCl), phosphoric acid (H₃PO₄), carbonic acid     (H₂CO₃), sodium dihydrogen phosphate (NaH₂PO₄), sodium dihydrogen     phosphate monohydrate (NaH₂PO₄.H₂O), sodium dihydrogen phosphate     dihydrate, sodium dihydrogen phosphate dehydrate, potassium     dihydrogen phosphate (KH₂PO₄), ammonium dihydrogen phosphate     (NH₄H₂PO₄), malic acid, acetic acid, lactic acid, citric acid,     malonic acid, succinic acid, glutaric acid, tartaric acid, oxalic     acid and their mixture. -   26. The method according to Item 22, wherein said basic solution is     selected from the group consisting of ammonia, ammonium hydroxide,     alkali metal hydroxide, alkali earth hydroxide, disodium hydrogen     phosphate (Na₂HPO₄), disodium hydrogen phosphate dodecahydrate,     disodium hydrogen phosphate heptahydrate, sodium phosphate     dodecahydrate (Na₃PO₄.12H₂O), dipotassium hydrogen phosphate     (K₂HPO₄), potassium hydrogen phosphate trihydrate (K₂HPO₄.3H₂O),     potassium phosphate tribasic (K₃PO₄), diammonium hydrogen phosphate     ((NH₄)₂HPO₄), ammonium phosphate trihydrate ((NH₄)₃PO₄.3H₂O), sodium     hydrogen carbonate (NaHCO₃), sodium carbonate Na₂CO₃, and their     mixture. -   27. The method according to Item 16, wherein step c-i) further     comprises allowing said first paste and said second paste undergoing     a hardening reaction in said mold. -   28. The method according to Item 16, wherein step c-i) further     comprises pressurizing said first paste and said second paste in     said mold after removing said one or more partition walls from said     mold to remove a portion of liquid from said first paste and said     second paste, so that a liquid/powder ratio of said first paste and     of said second paste decreases; and allowing said first paste and     second paste undergoing a hardening reaction in said mold. -   29. The method according to Item 16, wherein step c-ii) further     comprises allowing said intermediate undergoing a hardening reaction     in said first mold, and allowing said another one of said first     paste and said second paste undergoing a hardening reaction in said     second mold. -   30. The method according to Item 16, wherein step c-ii) further     comprises pressurizing said one of said first paste and said second     paste in said first mold to remove a portion of liquid therefrom     before the hardening reaction of said intermediate is completed;     allowing said intermediate undergoing a hardening reaction in said     first mold; pressuring said another one of said first paste and said     second paste in said second mold, so that a liquid/powder ratio of     said another one of said first paste and of said second paste     decreases; and allowing said another one of said first paste and     second paste undergoing a hardening reaction in said second mold. -   31. The method according to Item 28, wherein said pressuring is     about 1 to 500 MPa. -   32. The method according to Item 30, wherein said pressuring is     about 1 to 500 MPa. -   33. The method according to Item 16, wherein the immersing liquid is     an acidic aqueous solution, a basic aqueous solution, a     physiological solution, an organic solvent, or a substantially pure     water. -   34. The method according to Item 33, wherein the immersing liquid     comprises at least one of Ca and P sources. -   35. The method according to Item 33, wherein the immersing liquid is     a Hanks' solution, a HCl aqueous solution or an aqueous solution of     (NH₄)₂HPO₄. -   36. The method according to Item 16, wherein the immersing in     step d) is carried out for a period longer than 10 minutes. -   37. The method according to Item 16, wherein the immersing in     step d) is carried out for a period longer than 1 day. -   38. The method according to Item 16, wherein the immersing in     step d) is carried out at a temperature of about 10 and 90° C. -   39. The method according to Item 38, wherein the immersing in     step d) is carried out at room temperature. -   40. The method according Item 17 further comprising cleaning said     immersed shaped article before said drying; and heating the     resulting dried shaped article at a temperature between 50 and 500°     C.

Four different designs of prosthetic bone implants constructed according to the present invention are shown in FIGS. 1 a to 1 d. In FIG. 1 a, the prosthetic bone implant of the present invention has a dense cortical portion D1 in the tubular form and a porous cancellous portion P1 formed in the central through hole of the tubular cortical portion D1. Both the dense cortical portion D1 and the porous cancellous portion P1 are made of a hardened calcium phosphate cement having an apatitic phase as a major phase. In FIG. 1 b, the prosthetic bone implant of the present invention has a dense cortical portion D1 in the tubular form; a cylindrical porous cancellous portion P1 in the center of the tubular cortical portion D1; and an annular transitional portion P2 connecting the tubular cortical portion D1 and the cylindrical cancellous portion P0. The transitional portion P2 is made of a hardened calcium phosphate cement having an apatitic phase as a major phase, and a porosity gradient increasing from the lower porosity of the cylindrical cancellous portion P1 to the higher porosity of the tubular cortical portion D1, which may be formed in-situ during molding of two different two different CPC pastes, one of them having an additional pore-forming powder for forming the cylindrical cancellous portion P1, and another one being a regular CPC powder for forming the dense cortical portion D1. The porous cancellous portion P1 may be in the forms of isolated columns surrounded by the dense cortical portion D1 as shown in FIGS. 1 c and 1 d. Other designs are also possible in addition to those shown in FIGS. 1 a to 1 d.

A suitable method for preparing the prosthetic bone implant of the present invention includes placing a tubular partition wall 10 in a hollow cylindrical mold 20, as shown in FIG. 2 a; pouring a first paste comprising a calcium phosphate cement and a setting liquid in the annular cell and a second paste comprising the calcium phosphate cement, a pore-forming powder and the setting liquid in the central cell, as shown in FIG. 2 b; removing the partition wall and pressing the CPC pastes before hardening, as shown in FIG. 2 c, wherein a portion of the setting liquid is removed from the gap between the mold 20 and the press 30 and/or holes (not shown in the drawing) provided on the press 30. The CPC paste will undergo a hardening reaction to convert into apatitic phase. The hardened disk is removed from the mold and is subjected to surface finishing to expose the central portion hardened from the second paste, as shown in FIG. 2 d, followed by immersing in a bath of an immersing liquid as shown in FIG. 2 e, where the pore-forming powder is dissolved in the immersing liquid while the hardened CPC thereof gaining compressive strength. The immersing may last from 10 minutes to several days. The composite disk so formed is washed with water after being removed from the bath, and dried and heated in an oven to obtain the prosthetic bone implant as shown in FIG. 2 f. The heating is conducted at a temperature between 50 and 500° C. for a period of several hours to several days, which enhance the compressive strength of the cortical portion of the prosthetic bone implant.

An alternative method for preparing the prosthetic bone implant of the present invention from the same raw materials includes pouring the second paste in a first mold, pressing the second paste to remove a portion of the setting liquid from the second paste before the hardening reaction is completed, so that the liquid/powder ratio in the second paste decreases, and allowing the hardening reaction undergo in the mold for a period of time, e.g. 15 minutes starting from the mixing of the CPC powder, the pore-forming powder and the setting liquid, to obtain a cylindrical block having a diameter of 7 mm. Then, the cylindrical block is removed from the first mold, and placed in the center of a second mold having a diameter of 10 mm. The first paste is poured into the annular space in the second mold, and a press having a dimension corresponding to the annular shape is used to pressure the first paste to remove a portion of the setting liquid from the first paste before the hardening reaction is completed, so that the liquid/powder ratio in the first paste decreases. Again, the first paste will undergo a hardening reaction to convert into apatitic phase. The hardened cylinder having a diameter of 10 mm is removed from the second mold, followed by immersing in an immersing liquid, where the pore-forming powder contained in the second paste is dissolved in the immersing liquid while the hardened CPC thereof gaining compressive strength, to obtain the prosthetic bone implant of the present invention, as shown in FIGS. 3 a and 3 b. It is apparently to people skilled in the art that the prosthetic bone implant shown in FIGS. 3 a and 3 b can also be prepared by changing the sequence of the molding of the first paste and the second paste with modifications to the second mold used in this alternative method.

The following examples are intended to demonstrate the invention more fully without acting as a limitation upon its scope, since numerous modifications and variations will be apparent to those skilled in this art.

PREPARATIVE EXAMPLE 1 Preparation of TTCP Powder

A Ca₄(PO₄)₂O (TTCP) powder was prepared by mixing Ca₂P₂O₇ powder with CaCO₃ powder uniformly in ethanol for 24 hours followed by heating to dry. The mixing ratio of Ca₂P₂O₇ powder to CaCO₃ powder was 1:1.27 (weight ratio) and the powder mixture was heated to 1400° C. to allow two powders to react to form TTCP.

PREPARATIVE EXAMPLE 2 Preparation of Conventional TTCP/DCPA-Based CPC Powder (Abbreviated as C-PC)

The resulting TTCP powder from PREPARATIVE EXAMPLE 1 was sieved and blended with dried CaHPO₄ (DCPA) powder in a ball mill for 12 hours. The blending ratio of the TTCP powder to the DCPA powder was 1:1 (molar ratio) to obtain the conventional CPC powder. Particles of this C-CPC powder have no whisker on the surfaces thereof.

PREPARATIVE EXAMPLE 3 Preparation of Non-dispersive TTCP/DCPA-Based CPC Powder (Abbreviated as ND-CPC)

The TTCP powder prepared according to the method of PREPARATIVE EXAMPLE 1 was sieved and blended with dried CaHPO₄ (DCPA) powder in a ball mill for 12 hours. The blending ratio of the TTCP powder to the DCPA powder was 1:1 (molar ratio). The resultant powder mixture was added to a 25 mM diluted solution of phosphate to obtain a powder/solution mixture having a concentration of 3 g powder mixture per 1 ml solution while stirring. The resulting powder/solution mixture was formed into pellets, and the pellets were heated in an oven at 50° C. for 10 minutes. The pellets were then uniformly ground in a mechanical mill for 20 minutes to obtain the non-dispersive TTCP/DCPA-based CPC powder (ND-CPC). The particles of this ND-CPC powder have whisker on the surfaces thereof.

Dense Blocks

EXAMPLE 1 Effect of Immersion Time on Compressive Strength of CPC Block

To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 6 mm, and was compressed with a gradually increased pressure until a maximum pressure was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold. At the 15^(th) minute following the mixing of the liquid and powder, the compressed CPC block was immersed in a Hanks' solution for 1 day, 4 days, and 16 days. Each test group of the three different periods of immersion time has five specimens, the compressive strength of which was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) immediately following the removal thereof from the Hanks' solution without drying. The CPC paste in the mold was compressed with a maximum pressure of 166.6 MPa, and in the course of the compression the compression speeds were about 5 mm/min during 0˜104.1 MPa; 3 mm/min during 104.1˜138.8 MPa; 1 mm/min during 138.8˜159.6 MPa: and 0.5 mm/min during 159.6˜166.6 MPa. The measured wet specimen compressive strength is listed Table 1.

TABLE 1 Immersion time Compressive strength Standard deviation (Day) (MPa) (MPa) No immersion 37.3* 0.6  1 day 149.2 12.9  4 days 122.7 6.7 16 days 116.4 7.7 *This value was measured before the compressed CPC blocks were immersed in the Hanks' solution, and it was substantially the same for the compressed CPC blocks not immersed in the Hanks' solution measured a few days after the preparation.

It can seen from Table 1 that the compressive strength of the compressed CPC blocks is increased remarkably after one-day immersion in comparison with the non-immersed block, and declines a little for a longer immersion time.

EXAMPLE 2 Effect of Whiskers on Compressive Strength of TTCP/DCPA-Based CPC Block

The procedures of EXAMPLE 1 were repeated by using the C-CPC powder prepared in PREPARATIVE EXAMPLE 2 and the ND-CPC powder prepared in PREPARATIVE EXAMPLE 3. The maximum pressure used to compress the CPC paste in the mold in this example was 156.2 MPa. The results for one-day immersion time are listed in Table 2.

TABLE 2 Compressive strength Standard deviation CPC powder (MPa) (MPa) C-CPC (no whisker) 62.3 5.0 ND-CPC (with whisker) 138.0 8.2

It can be seen from Table 2 that the compressive strength, 62.3 MPa, of the immersed compressed CPC block prepared from the conventional CPC powder (no whisker) is about 1.7 times of that (37.3 MPa) of the non-immersed compressed CPC block in Table 1, and the compressive strength, 138.0 MPa, of the immersed compressed CPC block prepared from the non-dispersive CPC powder (with whisker) is about 3.7 times of that of the non-immersed compressed CPC block in Table 1.

EXAMPLE 3 Effect of Whiskers on Compressive Strength of TTCP-based CPC Block

Ca₄(PO₄)₂O (TTCP) powder as synthesized in PREPARATIVE EXAMPLE 1 was sieved with a #325 mesh. The sieved powder has an average particle size of about 10 μm. To the TTCP powder HCl aqueous solution (pH=0.8) was added according to the ratio of 1 g TTCP/13 ml solution. The TTCP powder was immersed in the HCl aqueous solution for 12 hours, filtered rapidly and washed with deionized water, and filtered rapidly with a vacuum pump again. The resulting powder cake was dried in an oven at 50° C. The dried powder was divided into halves, ground for 20 minutes and 120 minutes separately, and combined to obtain the non-dispersive TTCP-based CPC powder, the particles of which have whisker on the surfaces thereof. A setting solution of diammonium hydrogen phosphate was prepared by dissolving 20 g of diammonium hydrogen phosphate, (NH₄)₂HPO₄, in 40 ml deionized water. The procedures in EXAMPLE 1 were used to obtain the wet specimen compressive strength for one-day immersion time, wherein the maximum pressure to compress the CPC paste in the mold was 156.2 MPa. The results are shown in Table 3.

TABLE 3 Compressive strength Standard deviation CPC powder (MPa) (MPa) TTCP (no whisker) 79.6 8.8 TTCP (with whisker) 100 4.2

The trend same as the TTCP/DCPA-based CPC powder in Table 2 of EXAMPLE 2 can be observed in Table 3.

EXAMPLE 4 Effect of Molding Pressure on Compressive Strength of ND-CPC Block (in Low Pressure Regime: 0.09˜3.5 MPa)

The procedures of EXAMPLE 1 were repeated except that the maximum pressure used to compress the CPC paste in the mold was changed from 166.6 MPa to the values listed in Table 4. The period of immersion was one day. The results are listed in Table 4.

TABLE 4 Pressure for compressing the CPC paste in mold Compressive strength Standard deviation (MPa) (MPa) (MPa) 0.09 12.3 2.0 0.35 16.0 2.3 0.7 20.7 2.5 1.4 26.4 1.4 3.5 35.2 3.7

The data in Table 4 indicate that the compressive strength of the CPC block increases as the pressure used to compress the CPC paste in the mold increases.

EXAMPLE 5 Effect of Reducing Liquid/Powder Ratio during Compression of the CPC Paste in the Mold on Compressive Strength of ND-CPC Block

The procedures of EXAMPLE 1 were repeated except that the maximum pressure used to compress the CPC paste in the mold was changed from 166.6 MPa to the values listed in Table 5. The liquid leaked from the mold during compression was measured, and the liquid/powder ratio was re-calculated as shown in Table 5. The period of immersion was one day. The results are listed in Table 5.

TABLE 5 Pressure for L/P ratio (after a Standard compressing the CPC portion of liquid Compressive deviation paste in mold (MPa) removed) strength (MPa) (MPa) 1.4 0.25 26.4 1.4 34.7 0.185 75.3 3.9 69.4 0.172 100.4 6.8 156.2 0.161 138.0 8.2 166.6 0.141 149.2 12.9

The data in Table 5 show that the compressive strength of the CPC block increases as the liquid/powder ratio decreases during molding.

EXAMPLE 6 Effect of Post-Heat Treatment on Compressive Strength of CPC Block

The procedures of EXAMPLE 1 were repeated. The period of immersion was one day. The CPC blocks after removing from the Hanks' solution were subjected to post-heat treatments: 1) 50° C. for one day; and 2) 400° C. for two hours with a heating rate of 10° C. per minute. The results are listed in Table 6.

TABLE 6 Compressive strength (MPa) Standard deviation (Mpa) No post-heat treatment 149.2 12.9  50° C., one day 219.4 16.0 400° C., two hours 256.7 16.2

It can be seen from Table 6 that the post-heat treatment enhances the compressive strength of the CPC block.

Porous Blocks

EXAMPLE 7 Effect of KCl Content and Immersion Time on Compressive Strength of Porous CPC Block

To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. KCl powder in a predetermined amount was mixed to the resulting mixture by stirring intensively. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 6 mm, and was compressed with a gradually increased pressure until a maximum pressure of 3.5 MPa was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold. At the 15^(th) minute following the mixing of the liquid and powders, the compressed CPC block was immersed in a deionized water at 37° C. for 4 days, 8 days, and 16 days. The compressive strength of the specimens of the three different periods of immersion time was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) after the specimens were dry. The measured dry specimen compressive strength is listed Table 7.

TABLE 7 Dry compressive strength (MPa) Immersion time (Day) KCl/CPC ratio by weight 4 days 8 days 16 days 1 7.0 5.4 6.6 1.5 3.9 2.7 4.3 2 1.3 2.3 2.6

It can seen from Table 7 that the dry compressive strength of the porous CPC blocks decreases as the KCl/CPC ratio by weight increases.

EXAMPLE 8 Porosity and Compressive Strength of Porous CPC Blocks Prepared from Different Pore-forming Powders

The procedures of EXAMPLE 7 were repeated by using sugar, K₁, C₁₇H₃₃COONa and C₁₃H₂₇COOH instead of KCl. The immersion time was 14 days in deionized water. In the cases where the C₁₇H₃₃COONa and C₁₃H₂₇COOH were used, the CPC blocks were further immersed in ethanol for additional four days. The conditions and the results are listed in Table 8.

TABLE 8 Pore-forming powder S^(a)) C.S. (MPa)^(b)) Porosity (vol %)^(c)) Sugar 1 4.1 58.4 KI 2 4.3 62.2 KI 3 1.7 75.5 C17H33COONa 1 8.0 56.0 C13H27COOH 2 5.9 60.1 ^(a))S = Pore-forming powder/CPC by volume. ^(b))C.S. = dry compressive strength (hereinafter abbreviated as C.S.). ^(c))Porosity: Porosity (vol %) was measured by Archimedes' method, and calculated as in ASTM C830.

It can be seen from Table 8 that various powders which are soluble in water can be used in the preparation of a porous CPC block according to the method of the present invention.

Dual-Functional Block

EXAMPLE 9

To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. KCl powder in a ratio of KCl powder/CPC by volume of 2 was mixed to the resulting mixture by stirring intensively. The resulting paste was filled into a cylindrical steel mold having a length of 12 mm and a diameter of 7 mm, and was compressed with a gradually increased pressure until a maximum pressure of 3.5 MPa was reached. The maximum pressure was maintained for one minute, and then the compressed CPC block was removed from the mold at the 15^(th) minute following the mixing of the liquid and powders.

The resulting cylinder having a diameter of 7 mm was placed in another cylindrical steel mold having a diameter of 10 mm. To a setting solution of 1M phosphoric acid solution (pH=5.89) the ND-CPC powder from PREPARATIVE EXAMPLE 3 was added in a liquid/powder ratio (L/P ratio) of 0.4, i.e. 4 ml liquid/10 g powder, while stirring. The resulting paste was filled into the gap between said cylinder and said another mold, and was compressed with a gradually increased pressure until a maximum pressure of 50 MPa was reached. The maximum pressure was maintained for one minute. At the 15^(th) minute following the mixing of the liquid and ND-CPC powder, the CPC/KCl composite block was immersed in a deionized water at 37° C. for 4 days. KCl powder was dissolved in the deionized water, and a dual-functional CPC block having a porous CPC cylinder surround by a dense CPC annular block was obtained.

The compressive strength of the specimen was measured by using a AGS-500D mechanical tester (Shimadzu Co., Ltd., Kyoto, Japan) after the specimens were dry. The measured dry specimen compressive strength is 68.8 MPa.

The porosities of the porous CPC cylinder and the dense CPC annular block were measured by Archimedes' method, and calculated as in ASTM C830, after the dual-functional CPC block was broken intentionally, and the results are 74% and 30%, respectively.

X-ray diffraction pattern of the powder obtained by grinding the dual-functional CPC block shows broadened characteristic X-ray diffraction peaks of apatite at 2θ=25–27° and 2θ=20–35° w ith a scanning range of 2θ of 20–40° and a scanning rate of 1°/min. results indicate that the powder is a mixture of apatite and TTCP with apatite as a major portion. 

1. A method of forming a prosthetic bone implant comprising: adding a porous component paste to a first mold, wherein the porous component paste comprises at least one calcium phosphate cement combined with at least one pore forming agent and at least one setting agent to form a porous component; adding one or more porous components and a load bearing component paste to a second mold, wherein the load bearing component paste comprises at least one calcium phosphate cement combined with least one setting agent to form a hardened calcium phosphate article, the hardened calcium phosphate article comprising a load bearing component at least partially surrounding one or more porous components; immersing the hardened calcium phosphate article in one or more immersing liquids for a predetermined amount of time; and forming the prosthetic bone implant, wherein the prosthetic bone implant comprises the immersed hardened calcium phosphate article.
 2. The method of claim 1, wherein one or more porous components are substantially surrounded by the load bearing component.
 3. The method of claim 1, wherein immersion of the hardened calcium phosphate article is carried out at a temperature of from about 10° C. to about 90° C.
 4. The method of claim 1, wherein at least one immersing liquid comprises at least one source of calcium and least one source of phosphate.
 5. The method of claim 1, wherein at least one immersing liquid comprises Hank's solution.
 6. The method of claim 1, wherein at least one immersing liquid comprises substantially pure water.
 7. The method of claim 1, wherein the hardened calcium phosphate article is immersed in at least one immersing liquid for a time period sufficient to remove at least a portion of the pore forming agent from the hardened calcium phosphate article.
 8. The method of claim 1, wherein the hardened calcium phosphate article is immersed in at least one immersing liquid for at least 10 minutes.
 9. The method of claim 1, wherein the hardened calcium phosphate article is immersed in at least one immersing liquid for at least 24 hours.
 10. The method of claim 1, wherein the pore-forming agent comprises an inorganic salt.
 11. The method of claim 1, wherein the pore-forming agent comprises an organic salt.
 12. The method of claim 1, wherein the porous component paste comprises at least one calcium phosphate source, and the load bearing component paste comprises at least one calcium phosphate source.
 13. The method of claim 12, wherein at least one calcium phosphate source comprises alpha-tricalcium phosphate (α-TCP), beta-tricalcium phosphate (β-TCP), tetracalcium phosphate (TTCP), monocalcium phosphate monohydrate (MCPM), monocalcium phosphate anhydrous (MCPA), dicalcium phosphate dihydrate (DCPD), dicalcium phosphate anhydrous (DCPA), octacalcium phosphate (OCP), calcium dihydrogen phosphate, calcium dihydrogen phosphate hydrate, acid calcium pyrophosphate, anhydrous calcium hydrogen phosphate, calcium hydrogen phosphate hydrate, calcium pyrophosphate, calcium triphosphate, calcium phosphate tribasic, calcium polyphosphate, calcium metaphosphate, anhydrous tricalcium phosphate, tricalcium phosphate hydrate, or mixtures thereof.
 14. The method of claim 12, wherein the calcium phosphate source comprises tetracalcium phosphate, wherein at least a portion of tetracalcium phosphate particles comprises whiskers on the surface of the tetracalcium phosphate particles.
 15. The method of claim 1, wherein at least one of the setting agents comprises substantially pure water.
 16. The method of claim 1, wherein at least one of the setting agents comprises an acidic solution, and wherein the acidic solution comprises nitric acid (HNO₃), hydrochloric acid (HCl), phosphoric acid (H₃PO₄), carbonic acid (H₂CO₃), sodium dihydrogen phosphate (NaH₂PO₄), sodium dihydrogen phosphate monohydrate (NaH₂PO₄.H₂O), sodium dihydrogen phosphate dihydrate, sodium dihydrogen phosphate dehydrate, potassium dihydrogen phosphate (KH₂PO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄), malic acid, acetic acid, lactic acid, citric acid, malonic acid, succinic acid, glutaric acid, tartaric acid, oxalic acid or mixtures thereof.
 17. The method of claim 1, wherein at least one of the setting agents comprises a basic solution, and wherein the basic solution comprises ammonia, ammonium hydroxide, alkali metal hydroxide, alkali earth hydroxide, disodium hydrogen phosphate (Na₂HPO₄), disodium hydrogen phosphate dodecahydrate, disodium hydrogen phosphate heptahydrate, sodium phosphate dodecahydrate (Na₃PO₄.12H₂O), dipotassium hydrogen phosphate (K₂HPO₄), potassium hydrogen phosphate trihydrate (K₂HPO₄.3H₂O), potassium phosphate tribasic (K₃PO₄), diammonium hydrogen phosphate ((NH₄)₂HPO₄), ammonium phosphate trihydrate ((NH₄)₃PO₄.3H₂O), sodium hydrogen carbonate (NaHCO₃), sodium carbonate Na₂CO₃, or mixtures thereof.
 18. The method of claim 1, further comprising subjecting the hardened calcium phosphate article to one or more heat treatments.
 19. The method of claim 18, wherein the temperature of the heat treatments is from about 50° C. to about 500° C. 